The reactive impacts and harmonic produced by the non-linear load of the electric power system have severely threatened the safe operation and high quality power supply of the electric power system.
The hazards of the reactive power are:
(1) When the reactive power increases, the current grows as well as the apparent power of the power supply equipment increases accordingly; further more, it will lead to the dimension and specification augmentation of the starting equipment, control equipment and measuring instrument.
(2) The loss of the equipments and lines increases. The increase of the reactive power leads to the growth of the total current, so that the loss of the equipments and lines increases.
(3) The augmentation of the transformer and line drop leads to the severe power grid voltage fluctuation.
(4) The voltage fluctuation is mainly caused by the reactive fluctuation, which will lead to severe voltage fluctuation if it is caused by the impact reactive loads.
The hazards of the harmonic are:
(1) The harmonic make the equipments generate the additive harmonic loss, results in decreasing the efficiency of the power supply and distribution equipments as well as power consumption.
(2) The harmonic will influence the normal operation of various electric equipments and will lead to over voltage and over current, which give rise to the severe overheating of the transformer and the overheating of the capacitor and cable, the ageing of the insulation and curtailment of the lifetime.
(3) The harmonic will give rise to the local harmonic amplification in the public grid, even give rise to the parallel resonance and serial resonance, this phenomenon will lead to the equipment damage and safety misadventure, which deserves special attention.
(4) The harmonic will lead to the misconduct and operation refusal of the relaying protection and automatic devices.
Moreover, for the three-phase four-wire system, a great deal of 3n harmonic flows through the midline, which leads to the line overheating and easy component burning. The harmonic will influence the normal operation of the electric equipment, e.g., it will lead to the electric equipments mechanical vibration and noise, lifetime curtailment, even damage; it will make the measuring instrument accuracy decrease. The harmonic will severely interfere with the neighboring communication system, automation system and the systems composed of the micro electrics or computer equipments; slightly it will generate noise and decrease the operational quality, severely it will lead to the information loss and abnormal work. It is understood that the adverse effects of the higher harmonic are appeared in various respects, and effective measures shall be taken to suppress it.
In the electric power system, there are many methods to compensate the reactive power, but the Static Var Compensator using thyristor components has good performance, so in recent years, it grows stable in the world sphere and occupies the dominant position in the Static Var Compensator markets. So the Static Var Compensator (or SVC) usually means the static compensator using thyristor components, including the Thyristor Controlled Reactor (abbreviated as TCR), Thyristor Switched Capacitor (abbreviated as TSC) and the combination of these two devices (TCR+TSC), or the Thyristor Controlled Reactor and the Fixed Capacitor (abbreviated as FC), or Mechanically Switched Capacitor (abbreviated as MSC) and the mixed devices (for example, TCR+FC, TCR+MSC, etc.). Wherein, the TCR has the advantages of rapid response, reliable operation, step-less compensation, phase adjustment, power balance, wide application scope and cheap price. Therefore, the main electric equipment manufacturers in the industrial developed countries produce and actively promote this device, which is most widely used and is the mainstream for development. Currently more than 220 sets of the SVC with total capacity of 35000 Mvar are operating in the electricity transmission and distribution systems and more than 380 sets of the SVC with total capacity of 18000 Mvar are operating in the industrial sectors. The estimation shows that the SVC will have more developments in the electricity transmission and distribution fields and industrial users.
There are two methods to eliminate the harmonic in the power supply system: one is to filter the waves with passive LC filter or active power filter, the other one is to modify the harmonic source, for example, to improve the phase numbers of the convertor and adopt high power factor rectifier. Based on the harmonic absorption, the passive filter can compensate the reactive power and improve the power factor with features of simple structure and low cost; at the same time, due to its characteristics of easy maintenance, relative mature technology, design and manufacturing experiences, the passive filter solution is the most widely adopted major mean to suppress the harmonic and compensate the reactive power. But the passive power filter (PPF) has following defects: (1) the filter feature can be heavily influenced by the system parameters and working conditions, it is difficult for designers. Because the resonance frequency relies on the parameters of the components, it can only filter the major harmonic; the drift of the LC parameters will lead to the change of the filer feature, which makes the filter performance unstable. (2) The power grid parameter and LC may produce parallel resonance which leads to the component amplification of the harmonic and the power supply quality of the power grid decreases. (3) Sometimes it is difficult to coordinate the filter requirements, reactive power compensation and voltage adjustment requirement.
Another important trend to suppress the harmonic is to adopt the active power filter, because the active power filter can dynamically compensate the harmonic, reactive power and has negative sequence current without the resonance for the system; it produces better filtering effects than the passive electric power filter. But the active power filter is not used in China due to its large capacity and high cost.
There is always a close relationship between the reactive power compensation and harmonic suppression, both of their technology development and progress are mutually coordinated, the active filter can overcome the defects of passive filter in the real operation, which the compensation feature can be influenced by the power grid impedance variation and operation status, or it produces the harmonic amplification with the system, or even the parallel resonance. If a hybrid power filter is composed by combination of the passive filter and active filter, which can draw upon one another's strong points and compensate the weakness and has the advantages of these two filters, this solution is the hotspot for harmonic suppression research.
In the conventional compensation devices, the reactive power compensation, negative sequence current suppression and harmonic current suppression are carried out respectively and separately, which have not been omni-directionally streamlined according to a uniform mathematic model. The imperfection of the theory often leads to the conditions of taking one into consideration to the neglect of the other in the practice, even brings unfavorable influences among them. Because of adopting different devices to independently compensate, it is difficult to reach the purposes of complete compensation. The installation and maintenance working load is heavy and the cost-effective ratio is low.
It can be seen from the above introduction and analysis on the prior art that, currently there are no reports in China and overseas on dividing the LC filter branch of the passive filter into the pure tuned and additional inductance, then putting the active power filter parallel to the serial inductance.